Advents in the performance of microcomputer based electronics have resulted in dramatic increases in operating speeds of the logic switching circuits. Increased switching and operating speeds correspond to increased bandwidths of the electronic signals transmitted within the interior of a data terminal which become a significant source for electromagnetic radiation causing interference with the internal circuitry of the terminal itself and with other electronic devices operating within the vicinity of the data terminal. The electromagnetic radiation emitted at these higher frequencies may cause undesirable electromagnetic coupling between data paths resulting in crosschannel interference.
The amount of internally generated electromagnetic radiation must be limited to the guidelines and regulations set by governmental agencies such as the FCC in the United States and CISPR in European countries. Sources of electromagnetic radiation originating externally to the data terminal may also affect and interfere with the operation of the data terminal. In general the problems resulting from unwanted electromagnetic radiation are classified as electromagnetic interference (EMI).
Another significant contributor to operating malfunctions and faults in the ordinary course of terminal operations is electrostatic discharge (ESD). High density coulombic charge may accumulate on the terminal operator who then transfers the charge to the terminal when contact is made with the terminal. Other arbitrary sources of a concentrated electrostatic charge may be present in the operating environment with which the terminal may come into contact. The static discharge may generate electrostatic potential of a magnitude sufficient to interfere with the electronic signals within the terminal, cause a loss of stored data or even damage sensitive electronic components. Both the EMI and the ESD phenomena may cause valuable data to be lost or may cause the terminal to lock up or otherwise malfunction.
The intentional blocking of an electric, magnetic or electromagnetic field is referred to as shielding and is well known in the art. The concept of shielding is founded upon electromagnetic field theory through the application of Gauss' law. Gauss' law states that the net electric flux passing through any closed gaussian (mathematical) surface equals the net charge contained inside that surface divided by the permittivity of free space. Gauss' law is mathematically represented by one of Maxwell's equations, as applied to free space, which describes the net electric flux passing through any closed surface: ##EQU1## where E is total electric field intensity at any point on the surface, A is the surface area, Q is the electric charge contained within the surface, and .epsilon..sub.o is the permittivity of free space.
Thus the net electric flux contained within any gaussian surface is the surface integral of the electric field intensity on the surface which is also equal to the net charge within the surface divided by the permittivity of free space. Shielding employs a continuous metallic conductor as the gaussian surface. When an external field such as from an external source of electromagnetic radiation is applied to the surface the free electrons of the conductor are accelerated due to the externally applied field. The electrons are distributed in such a way that the electric field generated by the electrons opposes the externally applied electric field. The surface charge density then increases until the magnitude of the electric field set up by these charges equals that of the externally applied field, giving a net field of zero inside the conductor. Thus the conductor surface shields the interior of the conductor from the effects of externally applied electric fields.
In a good conductor, the time that it takes the conductor to reach electrostatic equilibrium where there is no net movement of electric charge is on the order of 10.sup.-16 s, which for most purposes can be considered instantaneous, that is when the frequency of the externally applied electric field is much less than 10 petaherz (10.sup.16 Hz). When the conductor is in electrostatic equilibrium the net charge within the conductor is zero.
If the net electric charge within the surface is zero, then the net electric flux will also be zero, and therefore the net electric field is zero everywhere inside the conductor. If the surface is a closed conductive surface, then no charge can enter or leave the surface; all of the charge is contained harmlessly on the surface of the conductor. Thus the net electric charge inside a gaussian surface is zero where the closed gaussian surface comprises a conductor.
The magnetic analog to Gauss' law describing the net electric flux within a gaussian surface is Gauss' law in magnetism. This law states that the net magnetic flux through a closed surface is always zero. Gauss' law in magnetism is stated as one of Maxwell's equations describing the net magnetic flux within a closed gaussian surface: ##EQU2## where B is the magnetic flux density, and A is the surface area. Implementation of magnetic shielding requires a magnetic material to block the lines of magnetic flux in a nonfluctuating magnetic field. The ideal material for shielding electromagnetic radiation exhibits both high electrical conductivity and magnetivity such that the magnetic permeability is greater than one, or a paramagnetic material.
No known shielding barrier stops 100 percent of the electromagnetic radiation that it is designed to block. The percent of energy that is blocked is called the shielding effectiveness. Shield continuity, the effectiveness of shielding, is a measure of the degree to which a shield confines or inhibits the electromagnetic field. This parameter depends upon the frequency of the electric field. Shielding becomes less effective with increasing frequencies of the electromagnetic fields. Therefore shield continuity is of increasing importance as the frequency increases.
The effectiveness of an electromagnetic shield is a function of the continuity, or physical completeness, of the barrier. The shield continuity factor is defined as the ratio of the actual shield conductor surface area to the total surface area which the shield area encloses, or: EQU C=A.sub.c /A.sub.t
where C is the shield continuity factor, A.sub.c, is the surface area of the shield conductor and A.sub.t is the total surface area which the shield encloses.
A solid metal enclosure, with absolutely no holes or gaps and with an excellent conductor provides 100 percent shielding continuity. If there are holes or gaps within the shielding, the effectiveness will be less than 100 percent. If it were possible to build a data terminal with a continuous, uninterrupted conductive shield, clearly no charge could enter or leave the terminal. Unfortunately, practical terminals have electrical input and output paths and openings required for displays, keys and the like. If the shield enclosure is not continuous, the opening should be very small compared with the wavelength of the electromagnetic field. Consequently, electrical signals entering and leaving the data terminal must be considered and treated to approximate the ideal closed surface shield.
Typical shielding methods known in the art coat the inside shell of a data terminal case fabricated of injection molded plastic with a paramagnetic electrically conductive material. However, the required openings in the terminal such as for input/output connectors, display and keypad prevent the shield from creating a completely closed surface. Therefore the shielding continuity factor will be less than one (i.e. less than 100 percent shielding effectiveness). Additionally the data paths interconnecting the internal circuitry itself are within the volume enclosed by the shield so that the shield is ineffective against mutual electromagnetic coupling of the internal circuitry.
As frequencies of operation increase in electronic circuits, circuit theory assumptions are no longer valid and field effects become significant. Thus in a standard circuit board layout of high frequency circuits field effect problems occur wherein all data paths are mutually coupled with all other data paths in the vicinity. Signals generated on one data path will stray to data paths in the nearby vicinity because of field effect coupling. Not only will erroneous signals be sent to the wrong components, but the stray field effects will cause an alteration in the impedances along the data paths. The altered impedances cause errors in data transmission and may lead to an increase in power consumption. Standard shielding will not prevent these problems which become more prevalent at greater bandwidths. Thus the bandwidths allowed by the prior art have an upper limit because of these field effects.
Another problem encountered with the prior art is the length of conducting cable that couples the circuitry with an entry point at the enclosing shielding surface. Traditionally, effective connection of electronic components requires mounting of the components on a printed circuit board assembly that generally also contains the processor, memory, and logic devices. Signals are typically routed and connected to the circuit board through some length of cabling which may unintentionally act as a transmitting antenna and radiate electromagnetic waves internally within the enclosing shielding surface to the circuitry of the data terminal thereby rendering the shielding ineffective against this source of EMI and causing interference with the operations of the data terminal. External input/output cables connected to the data terminal may also act as a receiving antenna by receiving external sources of EMI or ESD superimposed onto the signals carried on that cable and act as a transmission line by carrying the unwanted electromagnetic radiation to the internal components of the data terminal.
A possible solution to this problem is to provide treatment of the signal at the entry point of the terminal. However, such treatment is often either too costly or too impractical to implement in the constrained space of a small terminal. Treating signals which enter or leave a shield enclosure is often difficult requiring complex and specialized methods and processes to integrate shield enclosures with plastic components. In addition, signal treatment at the outer boundary opening will only reduce EMI from external sources but will not be effective against internally generated EMI. Further, computer type devices often have multiple access requirements for batteries, memory cards, and the like which further complicate achieving effective shielding.
High power and high speed circuits require a low impedance path for return current to minimize voltage potential differences between connected points in an electronic system. Ground is commonly the reference potential that logic circuits use for operation of digital circuits. Voltage potential differences along portions of the ground network may cause increased noise emissions and even circuit malfunctions. Typically a ground plane is employed to minimize ground potential by providing a low impedance circuit path on the ground network. Unfortunately the impedance of the ground plane is finite. As a result, an increase in the switching speeds of logic circuits results in increased switching currents thereby causing the existence of noise in the ground plane to be unavoidable. Consequently, the ground plane does not function as an effective shield as it may contain noise current that may be coupled to cables and other conductors that may radiate such noise as EMI. Designs which implement the ground plane as the same physical instrumentality as the shield invariably do not provide effective shielding at higher logic signal bandwidths.